Numerical study of a turbulent lobed jet with variable density (original) (raw)

Study of non-reactive isothermal turbulent asymmetric jet with variable density

Computational Mechanics, 2005

In the present study, the effects of inlet isothermal jet geometry on the mixing process with variable density have been investigated numerically. Four density ratios were considered, namely R ρ =7.2, 1.8,1 and 0.66 for He-air, CH4-air, Air-air and CO2-air mixtures respectively. The jets are produced through rectangular and elliptic nozzles with aspect ratio Ar = 2:1. The elliptic and rectangular nozzles have approximately the same exit area as the circular nozzle. A second-order Reynolds stress model (RSM) is used to investigate variable density effects in asymmetric turbulent jets. Comparative studies are presented for the calculations of the variables such as the longitudinal velocity, the longitudinal fluctuating velocity and the turbulent kinetic energy. The results indicate that the asymmetric geometry noticeably enhances mixing in comparison with the axisymmetric case. Typical phenomenon of 3D jets is observed. Keywords Asymmetric jet • Co-flowing • Turbulent • Slightly confined Nomenclature a Major axis of elliptic jet b Minor axis of elliptic jet Ar Aspect ratio (L / H) or a / b Ct Craya-Curtet parameter D eq Equivalent diameter of elliptic and rectangular jets F Average value of the mixture fraction f Scalar variance g i Gravitational acceleration in direction i G Production term due to buoyancy H Height of rectangular jet k Turbulent kinetic energy

The Effects of Scalloping Width and Position on Jet Mixing of Lobed Nozzles

Journal of Aerospace Technology and Management, 2015

AbstrAct: A series of geometric models of lobed nozzles with a central plug was created by different scalloping positions and widths. The shapes of lower and upper edges, cutting depth were kept unchanged. In this study, by the use of numerical simulation, the effects of scalloped width and position on the performance of jet mixing in the pumping condition were analyzed. The results indicated that, as the position of scalloped lower edge kept constant, the radial position of the accelerated mixing region of sidewalls did not change. The accelerated mixing region is enlarged as scalloped width increased, while the growth rate of enlarged region is less than the growth rate of scalloping width. When the scalloped region with the same width moved outward in the radial direction, the mixing rate of the region of the lobe outer edges increased. However, the distance for complete mixing of the primary stream in the core region was increased. It was also found that inward moving of scalloped lower edge enhanced the effect of strengthening streamwise vortices but induced more pressure loss.

Numerical Simulation of Effect of Lobed Nozzle on Temperature Mixing using Large Eddy Simulation

2014

A numerical study has been conducted to analyze the effect of nozzle exit geometry (lobed nozzle) on temperature mixing at the wake of jet. A lobed nozzle is a plate with corrugated edges and facilitates efficient mixing of co-flow streams. It accelerates the flow field and involves faster spreading of the jet. A three-dimensional flow field is produced by a subsonic jet. The computational study is conducted using the turbulence model Large Eddy Simulation (LES). The numerical model consists of a jet from the exit of a circular nozzle and a lobed nozzle (6 lobes). The exit area of both the nozzle exit geometry is kept equal for accurate analysis. A comparative study is taken into account to clearly depict the difference in temperature mixing, variation in turbulence and formation of vortical structures for the two different exit geometries. The velocity magnitude, static temperature and vorticity magnitude contours for the circular and lobed jet nozzle are produced and discussed. The results reveal the difference in the diffusion rates of the two nozzles. Lobed nozzle diffuses at a higher rate than that of circular nozzle. Design optimization of the lobed nozzle would lead to higher rates of efficient temperature mixing. The Large Eddy Simulation (LES) model is found appropriate to simulate the flow field for analysis of turbulent structure formation. The complete modeling and meshing has been carried out in ANSYS Workbench. For accurate predictions of the flow field, the flow has been simulated with the help of FLUENT.

Turbulent mixing in non-isothermal jet in crossflow

International Journal of Heat and Mass Transfer, 2015

Large eddy simulation (LES) of turbulent mixing in a non-isothermal jet in crossflow (JICF) configuration is conducted with a hybrid Eulerian-Lagrangian mathematical/computational methodology. In this methodology, a high-order finite difference (FD) multi-block method is used to solve the Eulerian filtered Navier-Stokes equations in a generalized coordinate system. The composition field is described by the filtered mass density function (FMDF) and its equivalent stochastic Lagrangian equations, which are solved by a Lagrangian Monte Carlo (MC) method. The consistency of the Eulerian and Lagrangian parts of LES/FMDF is established for isothermal and non-isothermal JICFs. It is demonstrated that the instantaneous and averaged scalar concentration and temperature fields as obtained from LES-FD and FMDF-MC data are very similar. The LES/FMDF results are also in good agreement with the available experimental data, indicating the accuracy and reliability of LES/FMDF model. The numerical results show that the variation in temperature has a significant effect on the flow and mixing in JICF. By decreasing the jet to crossflow temperature ratio for fixed jet and crossflow velocities, the jet penetration, the spreading rate, the entrainment and the mixing are considerably improved.

Mixing in Circular and Non-circular Jets in Crossflow

Flow Turbulence and Combustion, 2008

Coherent structures and mixing in the flow field of a jet in crossflow have been studied using computational (large eddy simulation) and experimental (particle image velocimetry and laser-induced fluorescence) techniques. The mean scalar fields and turbulence statistics as determined by both are compared for circular, elliptic, and square nozzles. For the latter configurations, effects of orientation are considered. The computations reveal that the distribution of a passive scalar in a cross-sectional plane can be single- or double-peaked, depending on the nozzle shape and orientation. A proper orthogonal decomposition of the transverse velocity indicates that coherent structures may be responsible for this phenomenon. Nozzles which have a single-peaked distribution have stronger modes in transverse direction. The global mixing performance is superior for these nozzle types. This is the case for the blunt square nozzle and for the elliptic nozzle with high aspect ratio. It is further demonstrated that the flow field contains large regions in which a passive scalar is transported up the mean gradient (counter-gradient transport) which implies failure of the gradient diffusion hypothesis.

Numerical Simulation of the Vortical Structures in a Lobed Jet Mixing Flow

43rd AIAA Aerospace Sciences Meeting and Exhibit, 2005

Numerical simulations of an incompressible jet mixing flow exhausted from a circular lobed mixer/nozzle were presented and validated against whole-field quantitative Dual-Plane Stereoscopic PIV (DP-SPIV) measurement results of the same flow. The numerical simulations were conducted using a Reynolds Averaged Navier-Stokes approach with a modest number of unstructured tetrahedral cells and four widely used turbulence models to predict the vortex structures in a lobed jet mixing flow. The predictability of the turbulence models to the lobed jet mixing flow were assessed and quantified based on quantitative comparisons of the numerical predictions with the DP-SPIV measurement results. It is found that the numerical simulations agree with the experimental measurements reasonably well in terms of streamwise vorticity and azimuthal (spanwise) vorticity. Although all the four turbulent models investigated over predict the magnitude of the turbulent kinetic energy significantly (about 50% to 130% over predicted), the numerical simulations are found to agree with the experimental results in predicting the locations of the regions with high turbulent kinetic energy, and the trends associated turbulent kinetic energy production and decay with downstream distance. It is found that the k-ε Realizable turbulence model provides the most accurate prediction of the lobed jet mixing flow among the turbulence models compared. Nomenclature

EFFECTS OF THE SHAPE OF A NOZZLE WITH CHEVRONS ON THE DYNAMICS OF TURBULENT IMPINGING JET

This work is a numerical study of a turbulent impinging jet issuing from a nozzle with chevrons. The Reynolds number based on the jet exit velocity and nozzle diameter is equal to 5000 corresponding to a low Mach number of 0.0057 at the nozzle exit. The main objectives of the investigation, inspired by the work of Violato et al.(Int. J. of Heat and Fluid Flow, 37, 2012) , are to highlight, from a fundamental point of view, the effects of the nozzle shape and the nozzle-to-plate distance on the mean parameters characterizing the dynamics of the flow in question. The nozzle configurations considered are a circular nozzle without chevrons and nozzles provided with 4 and 6 chevrons. The nozzle-to-plate distance ranges from 2 to 6 nozzle diameters. All the other flow conditions and geometrical parameters used in the different cases treated are identical. Interesting features of the flow are revealed by the obtained results of averaged three-dimensional fields of velocity and turbulent kinetic energy, particularly close to the wall. An attempt is made to bring additional insight into the phenomena in the free jet, the impingement region and the wall jet when using 4, 6 and no chevrons, for different nozzle-to-plate distances.

CFD Study of Effects of Geometry Variations on Flow in a Nozzle

Engineering Applications of Computational Fluid Mechanics, 2012

Reynolds-Averaged Navier-Stokes simulations have been performed to investigate the effect of nozzle geometry on the turbulence characteristics of incompressible fluid flow through nozzles at Reynolds number of approximately 50,000. Four nozzles have been considered: a baseline nozzle and three modified nozzles (extended, grooved and ringed). The flow in these nozzles has been simulated using different turbulence closure models, including Spalart-Allmaras, variants of k-ε and k-ω, and the Reynolds Stress Model (RSM). By comparison to experimental data, it is shown that the RSM produces more accurate results for the prediction of turbulent fluctuations. The presence of a ring significantly increases both the turbulence intensity and mean velocity at the exit, and requires a much higher inlet pressure to move the fluid through the nozzle. On the other hand, cutting a groove near the exit or extending the nozzle has little effect on the exit flow characteristics.

A CFD study of the effect of geometry variations on flow in a nozzle

Engineering Applications of Computational Fluid Mechanics, 2012

Reynolds-Averaged Navier-Stokes simulations have been performed to investigate the effect of nozzle geometry on the turbulence characteristics of incompressible fluid flow through nozzles at Reynolds number of approximately 50,000. Four nozzles have been considered: a baseline nozzle and three modified nozzles (extended, grooved and ringed). The flow in these nozzles has been simulated using different turbulence closure models, including Spalart-Allmaras, variants of k-ε and k-ω, and the Reynolds Stress Model (RSM). By comparison to experimental data, it is shown that the RSM produces more accurate results for the prediction of turbulent fluctuations. The presence of a ring significantly increases both the turbulence intensity and mean velocity at the exit, and requires a much higher inlet pressure to move the fluid through the nozzle. On the other hand, cutting a groove near the exit or extending the nozzle has little effect on the exit flow characteristics.

Aspect Ratio Driven Relationship between Nozzle Internal Flow and Supersonic Jet Mixing

Aerospace

This work attempts to connect internal flow to the exit flow and supersonic jet mixing in rectangular nozzles with low to high aspect ratios (AR). A series of low and high aspect ratio rectangular nozzles (design Mach number = 1.5) with sharp throats are numerically investigated using steady state Reynolds-averaged Navier−Stokes (RANS) computational fluid dynamics (CFD) with k-omega shear stress transport (SST) turbulence model. The numerical shadowgraph reveals stronger shocks at low ARs which become weaker with increasing AR due to less flow turning at the throat. Stronger shocks cause more aggressive gradients in the boundary layer resulting in higher wall shear stresses at the throat for low ARs. The boundary layer becomes thick at low ARs creating more aerodynamic blockage. The boundary layer exiting the nozzle transforms into a shear layer and grows thicker in the high AR nozzle with a smaller potential core length. The variation in the boundary layer growth on the minor and m...